Operational amplifiers, commonly referred to as Op Amps, are integrated circuit amplifiers that have two inputs, one commonly referred to as the positive or non-inverting input, and a negative input commonly referred to as the inverting input. The operational amplifier has a single output. Operational amplifiers form a portion of numerous circuit designs, and are typically implemented into circuits to form functional circuits such as inverters, comparators, and voltage regulators just to name a few.
Operational amplifiers fabricated on a semiconductor wafer are formed by numerous subcircuits including input stages, buffering stages, output stages. In each of these various stages there are defined components forming current sources, current mirrors, amplification stages and voltage rails, one rail adapted to receive a voltage source having a potential greater than the other rail, which is typically tied to ground but may have other voltage potentials depending on the implementation.
With specific reference to input stages for Op Amps, one relatively simple prior art input stage is shown in FIG. 1 and depicted at 10. FIG. 1 shows a first pair of transistors Q9 and Q10 along with transistors Q5 and Q6 forming on differential input, and transistors Q1 and Q2 forming another differential input. When the differential input formed by Q9, Q10, Q5 and Q6 is on, the transconductance (Gm) is half of that produced by the pair of transistors Q1 and Q2. This input stage is a good approach in that it can be easily implemented in a normal operational amplifier. The problem with this approach is that when the common mode range reaches the positive rail, the transconductance of the input stage halves.
There is desired improved input rail-to-rail input stage for operational amplifiers having the improved transconductance.